The present invention relates to a measuring arrangement, in particular on the basis of electrochemical sensors, for the an analysis of gas components in gas mixtures, in particular in exhaust gases of internal combustion engines, to.
It is known that exhaust gases of combustion engines, for example diesel engines, in addition to uncombusted fuel components and oxygen also contain nitrogen oxides and other gases. The composition of the exhaust gas is determined to a significant degree by the adjustment of the fuel-air mixture at which the internal combustion engine is operated. If, for example, fuel is present in stoichiometric excess, considerable quantities of uncombusted or only partially combusted fuel are present in the exhaust gas, while in the case of a stoichiometric excess of air oxygen in the fuel-air mixture, a correspondingly higher concentration of oxygen will be present in the exhaust gas. The analysis of the composition of the exhaust gas with a limit current sensor e.g. (Lambda sensor) is known as a method for setting an optimal fuel-gas mixture. The limit current sensor has a solid electrolyte arranged between two electrodes, with one electrode being exposed to the exhaust gas via a diffusion barrier. When a constant voltage is applied to the electrodes, a limit current will develop as a result of the difference in oxygen concentration at the two electrodes; this limit current is measured with a measuring arrangement and is evaluated, for example, for setting the fuel-air mixture at which the internal combustion engine is being operated.
A limit current sensor of this kind is described, for example, in German Patent No. 37 28 618. An electrode configured as a pump electrode is arranged in a diffusion channel, which is in contact on one side with the test gas mixture. Arranged in the diffusion channel is a diffusion barrier of such a design that an oxygen partial pressure corresponding to the voltage applied develops at the electrode in contact with the exhaust gas across the diffusion barrier.
German Patent Application No. 44 39 901 describes that a pump electrode for analyzing oxygen partial pressure can be combined with a second electrode for analysis of nitrogen oxides in gas mixtures. In this method the first electrode is covered by a gas-permeable membrane. A further possibility for analysis of additional gas components, in particular nitrogen oxides, in gas mixtures is described in the article by N. Kato, K. Nagakaki, and N. Ina in SAE 1996, pages 137 seqq. A particular disadvantage in these conventional methods was the reactivity of the nitrogen oxides and other gas components with parts of the electrode or of the catalyst.
A measuring arrangement for analysis of gas components in gas mixtures according to the present invention, in particular in exhaust gases of internal combustion engines, is composed of at least one electrochemical solid-electrolyte test cell, the cathode of which is covered with a selectively oxygen-ion-conducting layer that is separated from the cathode by an electrically insulating layer. In this manner any components of the exhaust gas such as sulfur oxides and nitrogen oxides, for example, cannot react with the cathode. In addition, the spatial separation makes it possible for electrically conductive materials to be used as an oxygen-ion-conductive layer without catalytic properties of the oxygen-ion-conductive layer being influenced by the electrical potential that is present on cathode 15 of the pump cell. In this manner oxygen can be selectively removed from the test gas so that other gas components can later be selectively analyzed.
By providing a second cathode that is not covered with an oxygen-ion-conductive layer, additional gas components within the gas stream can be selectively detected after the oxygen is pumped out. This applies in particular to the detection of nitrogen oxides or hydrocarbons or sulfur oxides which can be analyzed through appropriate selection of electrode for the second electrode, i.e., the second cathode. It is also advantageous that the pump capacity of the first oxygen pump cell can be set through the structure and thickness of the oxygen-ion-conductive layer. For example, it is conceivable for the method to be optimized such that all of the oxygen can be pumped off quickly and individual gas components of the gas mixture can be analyzed in the additional electrodes provided.
In one embodiment according to the present invention the oxygen-ion-conductive layer is composed of a mixed-conductive ceramic material. For example, compounds such as mixed-conductive metal oxides, preferably their structural variants doped with rare earth elements such as perovskites or elpasolites, but also cuprites, ferrites, and cobaltites, are used.
The oxygen-ion-conductive layer can be composed of catalytically active mixed metal oxide making it possible to use the pump cell as a lambda sensor since the equilibrium oxygen is determined.
Another embodiment of the present invention facilitates a use of mixed ion-conductive metal oxides which does not cause any change in the gas components, with the result that the free oxygen which has not reacted in equilibrium is measured. This makes it possible, for example, to diagnosis in a preferred, simple manner the quality, performance, and condition of the catalyst, for example, a catalyst for gas mixtures of internal combustion engines.
In another embodiment, it is possible to use mixed-conductive oxides which permit selective gas reactions on their surface providing a further selection possibility for various gas components, for example, nitrogen oxides in the gas mixture.
In another embodiment, the diffusion channel is arranged in a heat-resistant glass with the result that in a post-firing assembly process, all parts of the measuring arrangement can be constructed separately and joined together at low temperatures. In addition, the use of heat-resistant glass has the result that any gas components will not be able to react or will react only with difficulty with the material in which the diffusion channel is arranged, as is the case, for example, with conventional ceramics.
In another embodiment, the entire measuring arrangement can be heated so that the operating temperature is attained relatively quickly and thus even complex gas mixtures can be analyzed in a short time.
Another embodiment of the present invention also enables manufacturing the multilayer measuring arrangement in the post-firing process making it possible to use numerous material combinations which cannot otherwise be combined, for example in a co-firing assembly process as a result of the high temperature, since parts of the multilayer setup would decompose earlier. A co-firing assembly process is also possible in the case of the measuring arrangement according to the present invention with the proper selection of materials.